Formations in the earth are characterized by stress conditions which vary with depth and whose principal directions are generally vertical and horizontal. In the horizontal plane at any point, the horizontal stress reaches a maximum in one direction and a minimum at right angles to the maximum condition. Information concerning these maximum and minimum horizontal stress conditions is of substantial value in a variety of disciplines such as underground transportation systems, foundations of major structures, cavities for storage of liquids, gases or solids, and in prediction of earthquakes. Further, this information is essential in petroleum exploration and production, e.g. while drilling a well or borehole the information is useful for blowout prevention, in a completed well it is useful for evaluating hydraulic fracture treatment, and also in determining many critically important aspects of reservoir behavior, such as bulk and pore volume compressibility, permeability, direction of fluid flow, and reservoir compaction/surface subsidence.
Currently, the technique of hydrofracturing is often used to measure the least principal stress in the plane normal to the borehole axis, i.e., the normal plane. In hydrofracturing, the least principal stress in a normal plane is measured with a borehole injection test. While these injection tests are an accurate means of determining in-situ stresses and can be carried out at great depths, they are expensive, time consuming in that they require interruption of drilling to set borehole packers, and further, these tests are difficult to interpret.
In injection tests small volumes of fluid are pumped into small sections of the borehole, which are isolated by inflatable packers, with just enough pressure to create a small fracture. After each fracture of the formation, the pressure decline is measured as fluid leaks off. As long as the fracture is open, this pressure falloff should represent linear flow, and a plot of pressure falloff vs. the square root of time should be a straight line. Once the fracture closes, the pressure falloff is no longer linear and the slope of the pressure falloff vs. time plot will change. The point where this slope change occurs is interpreted to be the in-situ closure stress, which equals the minimum horizontal stress, for that depth.
The use of inflatable packers to isolate a test interval in a borehole is not only time consuming but can present another problem as these packers may cause unwanted fracturing of the formation. This unwanted fracturing would mean that the results of the fracturing tests are incorrect.
Accordingly, it is an object of this invention to improve fracturing of a selected location in a subterranean formation traversed by a borehole.
It is a more specific object of this invention to accomplish formation fracturing through a borehole which is filled with a fluid.
It is another more specific object to operate a downhole tool for formation fracturing without interrupting drilling operations.
It is yet another object to allow accurate calculation of principal horizontal stresses existing in the formation surrounding a vertical borehole.
It is yet another object to allow accurate calculation of principal stresses existing in the plane of the formation normal to an inclined borehole.